04 — Fasteners and assembly access

Bought hardware (screws, nuts, washers, inserts, pins, bearings, standoffs, rod ends) is chosen and placed with skills/mcmaster-parts: a real McMaster-Carr part number, its STEP model normalised into a known frame, and its specification table in the hardware catalogue, never an envelope.

Check tool access for every fastener at the step where it is installed

• Rule: For each fastener, check that a real tool can reach it and turn it at the point in the assembly where it is fitted, not only in the finished assembly.
• Incident: The first storage-stack design needed standoffs to be turned "inside narrow bores where no tool fits". The 6.6 mm column_bore in parameters.json holds the 5.5 mm AF hex standoff (6.35 mm across corners), which leaves no room for a 5.5 mm nut driver (about 8 mm OD). In the redesign, the standoffs hang from the part above: the male end passes up through that part's 2 mm ledge, the hex bears in a 2 mm counterbore, and the female end takes the next tray. Every standoff is turned while fully exposed. Earlier, the wall-mount check found that a 20 mm driver envelope collided with a gusset; the fix was projection 24 → 26 mm and hole offset 13 → 15 mm.
• Check: Sweep a tool envelope (driver diameter or wrench across flats plus swing) to each fastener head at its assembly step. Any intersection with plastic fails. mount-checks.json records driver_access with a 20 mm tool.
• Source: 2026-10-24T08:35 (summary of the 07:00 work); 2026-10-25T00:36; cad/parameters.json (interface_note); exports/mount-checks.json.

Choose the assembly direction that keeps fasteners exposed

• Rule: If a stack can only be fastened from one side, write the assembly order to match. Flipping the build (upside down) is acceptable.
• Incident: The storage stack is built upside down on the lid. Working from the top tray down: drive the drive screws in from below, route the leads, offer up the tray, and fit four M3 M-F standoffs through its ledges into the part above.
• Check: The README assembly steps state the order and which way up the assembly is. Each step names the tool and the access side.
• Source: 2026-10-25T00:36; README.md (Assembly step 10); cad/parameters.json (interface_note).

Compute screw length from the stack plus thread engagement

• Rule: Length = distance from the head seat to the start of the mating thread, plus the required engagement. Then check the result against the depth of thread actually available.
• Incident: The user defined it: housing bottom to the top of the AGX, plus "something like 8 threads into the standoff". The standoff thread starts at Z 40.76, and 8 threads of M3 (0.5 mm pitch) is 4 mm, so a head flush with the floor would need 44.3 mm. The agent pointed out that an M3 × 45 only fits if the standoff thread is at least 5.2 mm deep.
• Check: A table per fastener: seat Z, thread-start Z, engagement in threads and mm, the stock length chosen, and the minimum thread depth that length requires.
• Source: 2026-10-25T00:34, 00:36; cad/parameters.json (heatsink_thread_engagement_note).

Move the head seat to reach a standard screw length

• Rule: If the computed length is an uncommon size, move the seat (counterbore or countersink depth) so a stock length gives exactly the intended engagement.
• Incident: A 44.3 mm flat head is uncommon. The countersink seat was raised to Z 9.76 inside each Ø14 column, up a Ø7 pocket from the floor, so a standard M3 × 35 (DIN 7991 / ISO 10642) gives exactly 4.0 mm of engagement (9.76 + 35 = 40.76 + 4). This also puts the clamping cone in the solid column instead of the thin floor, and the base still sits flat.
• Check: Seat Z + stock length − thread-start Z = target engagement, within a stated tolerance. Recompute whenever a layer in the stack changes.
• Source: 2026-10-25T00:38; README.md (Assembly step 5); cad/parameters.json (heatsink_seat_note).

Print coupons for inserts and countersinks before the big parts

• Rule: Give each process-sensitive fastener feature a small test coupon, printed in the intended filament and profile before the main part.
• Incident: P11 is the M3 heat-set insert pilot coupon (nominal pilot 3.9 mm). P14 is the countersink coupon, used to check that the flat head sits at or below the floor. P12/P13 test the plate channel and key. The filament is unspecified, so insert retention and fits are not qualified.
• Check: Every insert pilot and countersink in the design has a matching coupon in the part list, and the assembly steps say to print it first.
• Source: 2026-10-24T07:41; README.md (Parts, Assembly step 1); cad/parameters.json (board_insert_pilot, lid_insert_pilot).

Limit screw penetration into drives to the SFF spec

• Rule: For screws into third-party devices, take the allowed thread penetration from the device spec. Pick the length so that (length − material under the head) falls inside that range.

• Incident: The drive hole patterns and penetration limits come from SFF-8301 Rev 1.9 and SFF-8201 Rev 3.4:

  • 3.5 in: a 6-32 × 3/16 in pan head through 1.9 mm of material, spec penetration 2.39–3.56 mm.
  • 2.5 in: an M3 × 4 button head through 2.3 mm, spec 0–2.5 mm.
  • 7 mm SSD side screws: penetration at most 2.0 mm (3.0 mm for thicker drives).

  By arithmetic (not stated in the session), the chosen screws penetrate about 2.86 mm (6-32) and 1.7 mm (M3 × 4), both inside the ranges.

• Check: storage-checks.json drive_fastening records the material under the head and the spec range per drive type. Add a computed-penetration field and assert that it is inside the range.

• Source: 2026-10-25T01:23; cad/parameters.json (drives, ssd_clip); exports/storage-checks.json.